(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer or a facsimile, which is provided with a two-component developing unit.
(2) Description of the Related Art
An image forming apparatus provided with a two-component developing unit is known, and such a developing unit is used by electrophotographic image forming systems, such as copiers, laser printers, and others. The two-component developing unit uses a two-component developing method which is effective and useful to provide a high speed image forming capability for such an image forming apparatus.
In the two-component developing method, a non-magnetic sleeve containing a magnet therein is used as a developing agent support. A developing agent, including powdered toner and carriers, is carried by the surface of the non-magnetic sleeve. A magnetic brush is formed on the surface of the non-magnetic sleeve, and the brush is placed adjacent to an image support of the image forming apparatus. The developing agent is retained and carried by the brush. On the surface of the image support, an electrostatic latent image is formed. An electric field between the image support and the sleeve is produced by supplying a developing bias voltage to the sleeve. The toner selectively adheres to the surface of the image support due to the electric field to form a toner image according to the latent image, and then the toner image is transferred to blank paper to form the copy.
For example, a laser printer of a certain type scans a laser beam across a positively charged rotating photoconductive drum. The photoconductive drum serves as an image support of the laser printer. The areas hit by the laser beam lose their charge, and the positive charge remains only where the copy is to be black. A negatively charged powdered toner adheres to the positively charged areas of the drum and is then transferred to blank paper to form the copy.
Generally, a unit-mass charge quantity Q/M (.mu.c/g) of the toner within the developing unit varies in a certain range depending on the surface characteristics of the toner and carriers and on the variations of the mass. Also, the toner is electrically charged when it contacts the carriers, and the unit-mass charge quantity Q/M of the toner varies depending on a non-driven period of the developing agent and on a mixing time of the developing agent. When the Q/M is too large, the optical density of the toner image becomes improperly low because of a firm adhesion between the toner and the carriers in the developing unit. When the Q/M is too small, or when the toner is reversely charged, the toner excessively adheres to the surface of the image support so as to produce a background smudge in the copy.
In order to eliminate the above-mentioned problems on the image quality, an AC bias developing method has been proposed. In the AC bias developing method, an AC (alternating current) bias voltage is superimposed to a DC (direct current) component of a developing bias voltage and it is supplied to the non-magnetic sleeve. It is known that the AC bias developing method serves to increase the developing capability. Further, it is known that reduction of the resistance of the carriers in the developing agent serves to provide an improved image quality and dot uniformity of the copy. Hereinafter, the developing capability is defined to be a ratio of the quantity of toner adhering to the image support (or the photoconductive drum) to the quantity of toner carried by the developing agent support (or the non-magnetic sleeve).
However, if the developing agent including reduced-resistance carriers is used, the Q/M of the toner considerably varies depending on the non-driven period of the developing agent and on the mixing time of the developing agent. After the developing agent is held in a non-driven condition over an extended period, the Q/M of the toner becomes too small. If the developing bias voltage wherein the AC bias voltage is superimposed to the DC component, is supplied to the non-magnetic sleeve with the developing agent in such a condition, a background smudge in the copy is likely to be produced.
Therefore, in order to achieve a good image quality of the copy, it is necessary to control the unit-mass charge quantity of the toner in the developing agent such that the Q/M of the toner is maintained at a constant value. If the Q/M of the toner is maintained at a constant value, it is possible to prevent the occurrence of a background smudge in the copy so that a good image quality can be achieved.
Generally, it is difficult to suitably control the charge quantity of the toner in the developing unit. Japanese Laid-Open Patent Application No. 8-44177 discloses an image forming apparatus having a developing unit provided with an auxiliary electrode. In the developing unit of the above publication, an AC bias voltage is supplied to the auxiliary electrode, and the supply of the AC bias voltage provides a mixing function of the developing agent and a control of the charge quantity of the toner, so as to reliably provide a good image quality of the copy. However, in the developing unit of the above publication, the developing bias voltage is not controlled in accordance with the charge quantity of the toner. After the developing agent is held in a non-driven condition over an extended period, the developing agent is carried by the non-magnetic sleeve and adheres to the surface of the image support before the time a start-up of the developing agent or the toner is achieved. Hence, in the developing unit of the above publication, a background smudge in the copy is likely to be produced when the developing agent is in such a condition, even if the AC bias voltage is supplied to the auxiliary electrode of the developing unit. Hereinafter, the start-up of the developing agent is defined to be a condition of the developing agent in which the quantity of charge of the toner therein apparently reaches an equilibrium condition.
Further, Japanese Laid-Open Patent Application No. 5-333673 discloses an image forming apparatus having a fixing unit provided with a warm-up time measuring device. The fixing unit is provided adjacent to the image support and serves to supply heat and pressure to a sheet having a toner image transferred from the image support, so that the toner image from the image support is stably fixed to the sheet. The warm-up time measuring device measures a warm-up time of the fixing unit in the image forming apparatus. When the warm-up time of the fixing unit measured by the warm-up time measuring device is larger than a reference time, a developing bias voltage supplied to the sleeve of a developing unit is controlled in accordance with a total number of copies counted by a copy counter of the image forming apparatus. More specifically, when the measured warm-up time of the fixing unit is too small, it is supposed that the start-up of the developing agent is not achieved. In such a condition, the DC component of the developing bias voltage supplied to the sleeve is increased. Also, the DC component of the developing bias voltage is increased as the total number of copies counted by the copy counter is increased to a given number. On the other hand, when the measured warm-up time of the fixing unit is larger than the reference time, it is supposed that the start-up time of the developing agent is achieved. In such a condition, the developing bias voltage supplied to the sleeve is held at a normal level.
However, in the image forming apparatus of the above publication, after the developing agent is held in a non-driven condition over an extended period (for example, one month), the developing agent is carried by the sleeve and adheres to the surface of the image support before a start-up of the developing agent. Hence, in the developing unit of the above publication, a background smudge in an initial copy is likely to be produced when the developing agent is in such a condition. Usually, the developing agent in the developing unit is mixed for a given time after the time a power switch of the image forming apparatus is turned ON in order to achieve the start-up of the developing agent. However, the developing agent, which has been held in a non-driven condition over an extended period, requires a mixing time longer than the given time because of a decrease of the Q/M of the toner.
A conceivable method for eliminating the background smudge with respect to the image forming apparatus of the above publication is that the developing agent, which has been held in a non-driven condition over an extended period, is mixed for a longer time after the power switch is turned ON. However, if the mixing time becomes long, the operator of the image forming apparatus must wait for the start-up of the developing agent over a long period. Hence, the operability of the image forming apparatus when the above-mentioned method is used is considerably lowered. It is difficult for the image forming apparatus of the above publication to provide both a good image quality and a speedy image formation when the developing agent is held in a non-driven condition over an extended period.